Method and apparatus for extruding metallic tubes



Sept 1, 170 H. WASSEN 3,525,119

METHOD AND APPARATUS FOR EXTRUDING METALLIC TUBES I Filed July 17. 1967 Fl 6. 1 MIN/0 wmxs uni

INVENTOR HQ? 411 4 1,

flg g j J fh' nq ATTORNEY United States Patent O 3,526,119 METHOD AND APPARATUS FOR EXTRUDING METALLIC TUBES Hans Wassen, Hilden, Germany, assignor to Stahlund Rohrenwerk Reisholz GmbH, Dusseldorf-Reisholz, Germany Filed July 17, 1967, Ser. No. 653,938 Claims priority, applicationgermany, July 20, 1966,

1, 4 Int. Cl. B21c 25/04 US. Cl. 72264 13 Claims ABSTRACT OF THE DISCLOSURE Tubular metallic blanks are extruded between a matrix and a floating mandrel to form tubes which are calibrated from without by the internal surface of a throat portion at the downstream end of the matrix and from within by an annual surface of constant circumferential length provided on a tip of the floating mandrel. The mandrel has a second portion which diverges in a direction awa from the tip and defines with a funnel-shaped upstream portion of the matrix an annular clearance of gradually diminishing cross-sectional area. The trailing ends of the blanks are subjected to a pressure which is below the flow limit corresponding to the temperature of the blanks but is high enough to etfect lengthwise advance.

BACKGROUND OF THE INVENTION The present invention relates to the production of pipes or tubes, and more particularly to a method and apparatus for conversion of tubular billets or blanks into calibrated tubes. Still more particularly, the invention relates to an extrusion method and apparatus which utilizes a floating mandrel or core.

It is already known to produce metallic tubes by extrusion of metallic blanks through a clearance defined by a matrix and a mandrel or core. As a rule, the mandrel is rigidly aflixed to a suitable holder or bridge so that it undergoes extremely high tensional stresses caused by frictional engagement with the mandrel of the blank. It happens quite frequently that the mandrel is actually torn away from its holder or tears into separate sections. Therefore, such extrusion apparatus are not suited for the production of tubes having a small internal diameter because the mandrel must have a relatively large crosssectional area in order to successfully withstand tension stresses which arise in response to frictional engagement with the material of the blank. As a rule, tubes produced in such apparatus must be subjected to a secondary treatment to impart thereto the final internal and/ or external diameter.

It was already proposed to install the mandrel in such a way that it can share lengthwise movements of the blanks. For example, certain known extrusion apparatus utilize mandrels which are yieldably afiixed to their holders; however, the frictional forces between the mandrel and the advancing material of the blank must remain within a relatively low range because, otherwise, the mandrel will be totally separated from the holder. It was further proposed to use a mandrel which is completely free to advance with the blank so that tensional stresses are greatly reduced but not eliminated, mainly because the rate of material flow in each zone of the clearance between the mandrel and the matrix is not the same, i.e., the rate of flow varies in longitudinally spaced sections of such clearance. Another serious drawback of the just described proposal is that the length of tubes. produced between a freely movable mandrel and the matrix is very short because the extrusion of a blank must be terminated when the mandrel has been moved to its foremost axial position. If the wall thickness of tubes is small, the apparatus can process only small blocks at a time; otherwise, the length of the mandrel would be well beyond a practical limit. Moreover, and since a residue remains upon completion of each extruding operation, the just described method of producing tubes by extrusion is quite uneconomical, not only as regards the output but also as regards the percentage of waste.

SUMMARY OF THE INVENTION It is an object of my present invention to provide a novel and improved apparatus for extrusion of tubes from tubular metallic billets or blanks which fully avoids the drawbacks of aforementioned conventional apparatus and which can be utilized for extrusion of thin-Walled or thick-walled tubes on a continuous basis.

Another object of the invention is to provide an apparatus which can be used for extrusion of metallic tubes with small internal diameter and wherein the holder for the mandrel may be dispensed with.

A further object of the invention is to provide an apparatus wherein the mandrel remains in desired axial position and is automatically centered in the matrix despite the fact that it merely floats in the matrix.

A concomitant object of the invention is to provide an extrusion apparatus wherein the mandrel is subjected to negligible tensional stresses so that the apparatus can be used in the production of tubes with very small internal diameters.

An ancillary object of the invention is to provide an apparatus which can extrude tubes to the desired diameter in a single pass and which can be used in the extrusion of tubes of an finite as well as of infinite length.

A further object of the invention is to provide an extrusion apparatus which operates with a minimum of waste in metallic material.

An additional object of the invention is to provide a novel method which may be practiced in the extrusion of metallic tubing by resorting to an apparatus of the above outlined character.

The method of my invention comprises the steps of advancing a tubular blank lengthwise between a surrounding annular calibrating surface and a surrounded annular calibrating surface which latter tapers gradually in the direction of advance of the blank and forms with the surrounding surface an annular clearance of diminishing cross-sectional area, conveying the thus deformed material of the blank along a surrounded annular surface of constant circumferential length to calibrate the resulting tube from within, and maintaining the trailing end of the blank under a pressure which is below the flow limit corresponding to the temperature of the blank but is high enough to effect lengthwise feed of the blank. The blank may be heated upstream of the annular clearance to a temperature which is below the recrystallization temperature of its material and the deformed material of the blank in the annular clearance may be heated above such recrystallization temperature. The heating to above recrystallization temperature can be carried out by induction heating and is terminated after the blank begins to move through the annular clearance. The blanks preferably consist of ferrous material and, if the material is a certain type of steel, are converted into tubes at a temperature which is below that required for conversion of alpha iron into gamma iron.

The apparatus comprises basically a matrix having a passage therethrough which is bounded by the aforementioned surrounding surface and through which the blanks are forced lengthwise with attendant reduction in cross section, and a floating mandrel received with clearance in the aforementioned passage and defining the two surrounded surfaces. The mandrel has a tapering portion and an end portion or tip of constant cross-sectional area. The configuration of the mandrel is such that stresses resulting from pressures exerted by the material of the blank balance or neutralize the stresses resulting from friction between such material and the mandrel. This insures that the mandrel remains in a selected axial position even though it is not connected to a support.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved extrusion apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary elevational view of a mandrel which can be utilized in the extrusion apparatus of my invention; and

FIG. 2 is a fragmentary axial sectional view of an ap paratus which employs mandrels of the type shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, there is shown a mandrel 7 having an end portion or tip 6 of constant cross-sectional area, a median portion 9 which tapers gradually toward the tip 6, and a rear end portion 8 of constant crosssectional area. In the illustrated embodiment, the portions 9 and 6, 8 are respectively bounded by smooth conical and cylindrical surfaces, i.e., the end portions 6, 8 resemble cylinders and the median portion resembles the frustum of a cone. The conicity of the median portion 9 has been exaggerated in FIG. 1 for the sake of clarity (compare with FIG. 2). In actual practice, the angle a/ 2 (between the axis of the mandrel 7 and a generatrix of the conical surface) is not in excess of degrees; preferably between 33.5 degrees. The length of the median portion 9 is a multiple of its maximum diameter.

FIG. 2 shows an extrusion apparatus having a hollow feed cylinder 1 provided with a through passage 1A and comprising several ring-shaped sections 1a, 1b, 1c, 1d which are heatable by induction heating. The foremost section It: is in abutment with an annular matrix 2 whose passage 2A registers with the passage lA. The rear portion 3 of the matrix 2 resembles a funnel and its internal surface 3a tapers toward the front end portion 6 of the mandrel 7. The larger-diameter end of the surface 3a is located at the level of the junction between the portions 8 and 9 of the mandrel 7. The throat 4 at the downstream end of the matrix 2 has an annular calibrating surface which determines the external diameter of the extruded tube 5. The taper of the internal surface 3a exceeds the taper of the external surface 9a on the median portion 9 so that the cross-sectional area of the gap be tween the portions 9, 3 decreases gradually toward the throat 4. The internal diameter of the tube 5 is determined by the cylindrical calibrating surface of the front end portion 6.

The feed cylinder 1 receives tubular metallic blanks 10 and 11. These blanks are fed lengthwise in response to pressure transmitted to the trailing end of the blank 10 (arrows 14) by a suitable press or the like, not shown. The pressure upon the blanks 10, 11 should be below the flow limit of the metallic material at the particular temperature. In other words, there should be no flow of the material of blanks 10, 11 in the interior of the feed cylinder 1. This is desirable because, were the material of the blanks free to flow in the sections 1a1d, it would reach the rear end portion 8 of the mandrel 7 and could cause the latter to change its axial position by friction. Such friction between the blanks and the rear end portion 8 could destroy the balance of frictional forces and pressure between the mandrel and the material of the blanks. Plastification of blanks upstream of the rear end face 15 of the mandrel 7 must be avoided under any and all circumstances because pressure exerted against the end face 15 would immediately change the axial position of the mandrel 7.

The blanks 10, 11 surround the rear end portion 8 of the mandrel 7 with a first annular clearance 13 and are received in the cylinder 1 with a second annular clearance 12. This allows for a reduction in the pressure which is being applied in the direction of arrows 14 to advance the blanks lengthwise. Also, the cylinder 1 is relieved of nearly all mechanical stresses so that it can stand long periods of use.

The matrix 2 is heated by induction heating in order to further raise the temperature of deformed material of the blanks 10, 11 which are preheated during travel through the feed cylinder 1. The heating of the matrix 2 can be terminated when the extrusion is in progress, i.e., the matrix will be heated solely during starting. Once the blanks begin to advance through and beyond the matrix, the latter need not undergo further heating provided, of course, that the cylinder 1 has raised the temperature of blanks to a desired value. The heat resulting from deformation of blanks then suffices to sustain the extrusion.

The configuration of the matrix 2 is such that the latter promotes the flow of metallic material in the region at the larger-diameter end of the median portion 9. This is the region where the upstream portion 3 of the matrix receives metallic material and where the pressure of material increases beyond the flow limit. The taper of the internal surface 3a is relatively small so that the portion 3 insures satisfactory flow of metallic material without necessitating very high pressures (arrows 14).

Pressures required to advance the blanks 10, 11 lengthwise will be lower if the blanks are fed at a relatively low speed. As a rule, the rate of feed will be less than in presently known extrusion apparatus wherein extremely high speeds are desirable in order to reduce the period of contact between hot metal and the mandrel, i.e., to avoid excessive heating of the mandrel. In the apparatus of my invention, the period of contact between the mandrel and the material of the blanks is very long, especially in continuous extrusion of endless tubing and when the blanks are being fed at a low speed. Therefore, the material of the mandrel 7 is selected with a View to withstand the temperatures which develop in the interior of the feed cylinder 1 and matrix 2. The material of the mandrel 7 and matrix 2 is preferably hard metal. Since such material is attacked by oxygen in the air, the apparatus preferably includes an envelope (indicated at 16) which is filled with protective gas. The delivery of fresh blanks is carried out in the customary way, e.g., by a system of locks.

Even though the rate at which the blanks are being fed into the space between the mandrel 7 and matrix 2 is normally less than in conventional extrusion apparatus, the output of my apparatus is not less and is often higher than in such conventional systems. This is due to the fact that the times required for setting up a conventional apparatus for a fresh operation (including retraction, cooling and eventual replacement of the mandrel and insertion of fresh blanks) are much longer. In such conventional apparatus, the setup consumes more time than the extrusion; in many instances the setup takes up a period of time which is a multiple of the time required for conversion of a blank into a tube. The apparatus of my invention can operate continuously so that its output per shift is normally higher or not less than the output of known extrusion apparatus.

Referring again to FIG. 1, when the apparatus is in use, metallic material surrounding the median and front end portions of the mandrel 7 is subjected to what might be called hydrostatic pressure, i.e., pressures are nearly identical in all directions. A pressure P acts upon each unit surface area of the median portion 9. This pressure P has a component P acting upwardly in the axial direction of the mandrel 7. Furthermore, friction between the material around the mandrel 7 and a unit area of the surface on the median portion 9 produces a force ,u.P (wherein a is the coefficient of friction) having a component P acting downwardly in the axial direction of the mandrel.

Each unit area of the surface on the front end portion 6 of the mandrel 7 is subjected to a pressure P which produces a frictional force ,mP' acting downwardly in the axial direction of the mandrel. As stated before, the rear end portion '8 of the mandrel 7 normally does not come in contact with the material of the blanks.

The mandrel 7 will float (i.e., it will not change its axial position) if all forces which act in the axial direction of the mandrel are in equilibrium. Such equilibrium will depend on the coefficient of friction and on the angle alpha at the apex of the imaginary extension of the surface on the median portion 9. If the influence of the front end portion 6 upon the equilibrium were eliminated, the fric tion angle would have to equal a/2. Under such circumstances, each point on the surface of the mandrel would be subjected to balanced stresses and the force tending to move the mandrel axially would equal zero. The magnitude of the angle alpha can be readily determined by simple experimentation. This magnitude will depend on the material of the mandrel, on the material of blanks, and on the type and quantity of lubricant. It was found that the angle alpha need not exceed ten degrees and is usually between six and seven degrees.

The balance of forces acting upon the mandrel in actual operation is stable. Once the extrusion is in progress, the mandrel 7 retains its axial position and, even if displaced due to unforeseen disturbances, automatically returns to an optimum position. Relatively small axial displacements of the mandrel from such optimum position have no appreciable efiect on the wall thickness of the tube 5. This is due to the fact that the front end portion 6 is a body of constant cross-sectional area. Thus, as long as the end portion 6 remains in the throat 4 of the matrix 2, the internal diameter of the tube 5 will not change at all. It was further found that the mandrel 7 centers itself automatically, i.e., that its axis coincides with the axis of the matrix 2, and that the mandrel remains in such position of axial alignment with the matrix.

The mandrel 7 preferably consists of sintered hard metal or stellite. Such materials exhibit a very satisfactory hardness at temperatures in the region of and above 800 C. As stated before, the operation is preferably carried out in an atmosphere of protective gas.

The heating action of the feed cylinder 1 is such that the blanks 10, 11, etc., are heated to slightly below recrystallization temperature. The recrystallization temperature is reached and exceeded when the material of a blank passes through the upstream portion 3 of the matrix 2. As stated before, the matrix will be heated inductively at the beginning of an extrusion operation but the heating action is terminated when the extrusion is in progress. Steel pipes are preferably extruded at a temperature which is below that necessary for conversion of alpha iron into gamma iron. In such temperature range, the resistance to deformation of blank material is at a minimum. Of course, this applies only to such steels wherein a conversion of alpha into gamma iron will take place. In extrusion of other types of steel, the temperature is raised to the upper limit which can be withstood by the components of the extrusion apparatus.

A very important advantage of my apparatus is that it can be used for continuous or intermittent extrusion of seamless tubes with a small internal diameter and that such tubes can be produced by passing the material only once through the extrusion zone. This is due to the fact that the mandrel 7 is subjected to negligible axial stresses and, therefore, its front end portion 6 which calibrates the tube 5 from within can have a very small cross-sectional area. The tubes can be produced continuously or infinite lengths. The output of the apparatus is very high, especially in the production of continuous tubing, because the time wasted for setting up is much less than in conventional apparatus. The surface finish of tubes produced in a protective atmosphere is so satisfactory that such tubes require no further treatment, and the wall thickness of tubes is uniform all the way around as well as in the axial direction. This is attributed to the self-centering action of the mandrel and to the fact that the mandrel tends to remain in an optimum axial position without suspension. This reduces the cost of tubes, particularly of such types of tubes which are bounded by cylindrical surfaces and wherein the flow of material is not impeded by grooves, protuberances and/or other irregularities.

It is clear that the apparatus of my invention is equally suited for the extrusion of tubes with other than circular cross-sectional outline. The apparatus can be used for extrusion of a wide variety of hollow tubular products. All that counts is to employ a mandrel wherein a portion of gradually diminishing cross-sectional area is located upstream of a front end portion with constant cross-sectional area. For example, the median portion 9 of the mandrel 7 could resemble a slender truncated pyramid and the front end portion 6 could resemble a multifacetted pin or post of polygonal cross-sectional outline.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of my contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

1. Apparatus for continuously extruding tubular metallic blanks into tubes, said apparatus comprising, in combination, a matrix having a passage gradually tapering from one toward the other end thereof; a floating mandrel received with clearance in said passage and having a front end portion of constant cross-sectional area located at said other end and a second portion which is more distant from said other end than said front end portion and which has a peripheral surface which tapers toward said front end portion at an angle of less than 10 and means for applying to the trailing end of a blank received in said passage a pressure which is below the flow limit corresponding to the temperature of the blank but which is high enough to efiect lengthwise advance of the blank from the one to the other end of said matrix.

2. Apparatus as defined in claim 1, wherein the length of said second portion, as considered in the direction of lengthwise travel of blanks through said passage, is a multiple of the maximum transverse dimension of said second portion.

3. Apparatus as defined in claim 1, wherein said angle is between 6 and 7 degrees.

4. Apparatus as defined in claim 3, wherein said man drel is centered in said passage solely by the material of the blanks.

5. Apparatus as defined in claim 1, wherein the material of said mandrel is selected from the group consisting of sintered hard metal and stellite alloys.

6. Apparatus as defined in claim 1, further comprising an enclosure filled with protective gas and accommodating said mandrel and said matrix.

7. Apparatus as defined in claim 1, wherein said one end of said passage is located at the level of that end of said second portion which is remote from said front end portion.

8. Apparatus as defined in claim 1, wherein said front end portion is constituted by a short cylinder and wherein said second portion is constituted by thefrustum of a cone.

9. Apparatus as defined in claim 1, wherein said mandrel further comprises a rear end portion of constant cross-sectional area surrounded ove the whole length with clearance by successive blanks, said matrix having a throat portion surrounding said front end portion of the mandrel to calibrate the tube from without, and further comprising a heated tubular feed cylinder surrounding with clearance the blanks advancing lengthwise of and around said rear end portion of the mandrel.

It). A method of converting tubular metallic blanks into tubes comprising the steps of continuously advancing a tubular blank lengthwise in one direction between a surrounding annular surface and a surrounded annular surface which latter tapers gradually in the direction of advance of the blank and forms with the surrounding surface an annular passage of diminishing cross-sectional area; conveying the thus deformed material of the blank in said direction along a second surrounded annular surface of constant circumferential length to calibrate the resulting tube from Within; and maintaining the trailing end of the blank under a pressure which is below the flow limit corresponding to the temperature of the blank but is high enough to effect lengthwise advance of the blank.

11. A method as defined in claim 10, further comprising the steps of heating the blank upstream of the passage to a temperature which is below recrystallization temperature of its material, and heating the blank above such re- 8 crystallization temperature during travel through said clearance.

12. A method as defined in claim 11, wherein said second heating step is carried out by induction heating and is terminated after the blank begins to move through said passage.

13. A method as defined in claim 10, wherein the blanks consist of steel and are converted into tubes at a temperature which is below the temperature required for conversion of alpha iron into gamma iron.

References Cited UNITED STATES PATENTS 2,355,734 8/1944 Katz 72-283 3,342,055 9/1967 Blankenship 72-38 FOREIGN PATENTS 87,123 6/1896 Germany. 805,617 12/1958 Great British. 928,893 6/ 1955 Germany.

FOREIGN PATENTS STAL: June 1963, Drawing Medium-Grade Carbon Steel Tubes on a Floating Mandrel, Orro, et al., pp. 446- 469.

LOWELL A. LARSON, Primary Examiner 

